In the field of fluid mechanics, engineers have long sought a velocity measurement technique that avoids the complications associated with intrusive probes. In the present application, an acoustic velocity measurement technique, based upon sonic anemometry, is presented which can be used for high subsonic Mach number measurements. Contrasting with existing acoustic techniques, this method has been developed for and demonstrated in flow Mach numbers above 0.3. To facilitate further discussion of this technique, and its improvements over current acoustic methods, an overview of the state of the art is first provided.
In practice, two fundamental acoustic techniques exist for non-intrusively measuring flow velocities. These two techniques are Acoustic Doppler Velocimetry (“ADV”) and Sonic Anemometry (“SA”). The major difference between these two techniques is the domain in which they are analyzed. ADV, for example, uses the Doppler Shift Effect, which is analyzed in the frequency domain, to characterize flow velocity. To perform this measurement, an acoustic pulse with a known frequency is first emitted into the flow field. Particles, moving in the stream-wise direction, then reflect the acoustic signals back to a detector. The reflected signal, which now has a Doppler shifted frequency, is then compared with the initial frequency to determine the particle's velocity as given by equation (1) below.
                              V          p                =                              c            *                          f              d                                            2            ⁢                                                  ⁢                          f              o                                                          (        1        )            
Above, Vp is the velocity of the particle, c is the speed of sound in the medium, fd is the Doppler shift frequency, and fo is the initial frequency.
SA, on the other hand, uses time of flight measurements in order to determine the flow's velocity using the time domain, with sensitivity to the flow velocity exhibited in the line integral equation for the time of flight of an acoustic wave.
                    t        =                  ∮                                                                  d                ⁢                                                                  ⁢                                  s                  _                                                                                  c              +                                                                    V                    _                                    ·                  d                                ⁢                                                                  ⁢                                  s                  _                                                                                        (        TOF        )            
To perform an SA measurement, two acoustic path measurements are made between emitter and receiver pairs. The first measurement set follows a downstream acoustic propagation path (V·ds>0), and the second set follows the upstream propagation path (V·ds<0). Using the fact that the acoustic signal takes longer to propagate in the upstream direction due to the reduced denominator in equation (TOF), the medium's velocity can be determined “relatively independently of the flow properties (spatial and time variations, density, temperature, etc.).” By using precisely the same path but with opposite wave directions, the SA velocity equation is shown as equation (2) below.
                              V          d                =                              L            2                    ⁢                      (                                          1                                  t                  d                                            -                              1                                  t                  u                                                      )                                              (        2        )            
In equation (2), Vd is the velocity of the medium in the direction parallel to the component distance vector, L is the distance between the transmitter and receiver (should be approximately the same for upstream and downstream measurements), td is the downstream propagation time delay, and tu is the upstream propagation time delay.
While both SA and ADV measure velocity using acoustics, there are drawbacks of using ADV which make it unfeasible in a high noise environment. Perhaps the greatest among these is the need for entrained particles. These particles, if not readily available in the flow, must be introduced into the flow stream—impractical for in situ applications. The other significant drawback of using ADV is the relatively low signal-to-noise ratio (SNR) inherent in scattered acoustic signals. In order to measure velocities using ADV, an optimal SNR of 10 dB must be obtained. Other research has found that a 20 dB SNR assures reliable velocity measurement. This signal-to-noise ratio would be difficult to obtain in a noisy environment, typical of high subsonic Mach number jets, because SNR is a function of entrained particle diameter and concentration. For the given reasons, emphasis in this article will be placed on the time delay based velocity measurement approach instead.
The existing SA technique also poses several challenges that make immediate implementation in high subsonic Mach number jets difficult. Perhaps the most pertinent challenge is that this technique has only been proven for low Mach numbers. A survey of the literature reveals that reported applications are restricted to velocities less than 100 m/s, while reported uncertainties are widely varied. One study reports a typical sonic anemometer's velocity range is ±30 m/s with accuracies in the range of +0.02-0.05 m/s. Kaijo-Denki type DA310 devices, on the other hand, have been used to measure wind velocities up to 60 m/s with an accuracy of ±1% of the measurement values. In an experimental setting, Mylvaganam et al. performed an investigation in gaseous flows from 0 m/s to 100 m/s, but were only able to report velocity measurements up to 70 m/s with a 3% uncertainty reported across the measurement span. Assuming 20° C. ambient conditions, the 100 m/s limitation of the aforementioned SA device corresponds to a Mach number of 0.3.
This Mach number limitation is, in large part, a product of the upstream acoustic propagation requirement. Although this implementation results in optimized uncertainties by maximizing the velocity-dependent time delay difference [c.f. equation (2)], signal degradation due to propagation curvature and impedance grows with Mach number. This argument is supported by an investigation performed by Tack & Lambert (1965). The overall effect of this increased acoustic attenuation is a decrease in SNR which has a significant negative impact on time delay estimation accuracy. In order to successfully perform a non-invasive acoustic velocity measurement in a high subsonic Mach number, SNR must be maximized to ensure accurate time delay and, ultimately, velocity estimation.
The present application discloses one or more of the features recited in the appended claims and/or the following features which, alone or in any combination, may comprise patentable subject matter.